Transmitter circuitry in mobile communication devices, such as mobile phones, typically includes upconversion circuitry to, for example, shift the frequency of a baseband signal to a radio frequency for transmission. In a typical transmitter architecture, an upconverted signal is amplified by a driver amplifier (DA) within a radio frequency integrated chip (RFIC) device. The amplified signal may then filtered by an off-chip bandpass filter (BPF), which is, typically, implemented as surface acoustical wave (SAW) filter, or a film bulk acoustic wave (FBAR) filter due to their low insertion losses and high quality factors. These filters typically convert electrical signals to a mechanical wave in a device constructed of a piezoelectric crystal or ceramic. This mechanical wave may then be delayed as it propagates across the device, before being converted back to an electrical signal for transmission through an antenna. Here, such a BPF may be tailored to reduce receiver band noise. However, as integration of a piezoelectric with an RFIC may have practical limitations, such a BPF is typically implemented as an off-chip component, which increases costs and bill of materials (BOM), and occupies significant board area. Hence, there has been a trend to remove an off-chip BPF (e.g., a SAW filter) from the transmission path between an RFIC device and a power amplifier (PA), especially in a multi-band transmitter.
In a particular application, a transmitter may be implemented in a mobile device (e.g., mobile phone for communication with a base station). Here, such a mobile device may transmit maximum power in a narrow range of output power. An upper end of this range may be limited by user safety (e.g., to avoid excessive exposure to RF radiation). A lower end of this range may be set according to data rate/throughput requirements. A transmitter typically includes an RFIC device and a power amplifier. A power amplifier gain may vary from one particular manufactured device to another. Gain variation may also arise from other factors including changes in the frequency at which the mobile device operates and changes in the temperature of the mobile device. In response to a change in power amplifier gain, a mobile device may increase or decrease output power from an RFIC device so that a maximum output power from the mobile device is restricted to a desired range. Typically, a level of noise relative to output power of an RFIC device degrades by 0.5 dB for every 1.0 dB decrement in power. Since the power from an RFIC device and gain of the power amplifier may be adjusted to ensure that the output power is within a target range, degradation in a relative level of noise at the RFIC may result for an equivalent degradation in an absolute level of noise at the mobile device output. Such a degradation (increase) in noise at a mobile device transmitter output may lead to increased noise leaking into a receiver through a duplexer. Such noise leaking through the duplexer may degrade the performance of the mobile device receiver. Hence, a BPF is typically implemented to reduce RxBN. The problem is compounded in GSM networks where the requirements of the relative level of noise at the mobile device output are very stringent. Absolute noise requirements may be more or less the same as in CDMA1x or WCDMA networks. However, since the output power is higher, the required relative level of noise may be more difficult to achieve. A typical GSM mobile device may employ a saturating power amplifier with limited gain variation. Newer GSM mobile devices may employ a “small signal polar” design that usually is accompanied with much larger power amplifier gain variation. The two factors—increasing gain variation in power amplifier and more difficult relative noise requirements lead mobile device manufacturers to artificially boost an output power of an RFIC device first. Relative noise from an RFIC device may be improved at these boosted powers. The power may then be attenuated by an external attenuator to ensure that the power amplifier is not over-driven. Such an external attenuator may enable a reduction in power without impacting a relative level of noise. It should be noted that in GSM networks noise in a receive band may not desensitize a mobile device receiver because this kind of transmitter only has either the transmit or receive path operational at a given time instant. Instead, the noise from the transmitter of a first mobile device may desensitize the receiver of a second mobile device which may be in close physical proximity to the first device.